EP1955100B1 - Optical instrument comprising an input cavity wherein is arranged a mirror - Google Patents

Abstract

The instrument (1) has a cavity (2) comprising an inner rigid envelope (20) around a primary mirror (3) comprising an active surface subjected to instantaneous variations of an incident radiant flux. The envelope is constituted of a material e.g. aluminum or beryllium, having high thermal inertia to dampen the instantaneous variations. The envelope extends on a cavity part that is near the mirror, where the part moves from the mirror to a distance lesser than the total length of the cavity.

Description

The invention relates to an optical instrument comprising an entrance cavity in which is placed a so-called primary mirror mirror.

The invention applies to any optical instrument comprising a mirror requiring a very high thermal stability, to limit the thermoelastic deformations, especially over short periods, for example one to two hours.

The invention is particularly but not exclusively applicable to optical instruments used in the spatial field such as optical instruments embedded on satellites (typically telescopes).

Indeed, certain optical instruments such as telescopes require a very high geometric stability at room temperature of their primary mirror, both in the long term and in the short term.

With the use of new ceramic materials (based on silicon carbide: CeSiC, SiC, ...) for the production of mirrors, this constraint is reflected inter alia by a high thermal stability in terms of gradient variation in the mirror thickness and in terms of temperature fluctuation of the active face. These so-called primary mirror mirrors, located in an input cavity of the instrument, are directly or indirectly subjected to variations in external fluxes (solar, terrestrial or albedo) in the orbit, and throughout the year.

Until now, the thermal regulation of such mirrors has been ensured by active radiative type rear-panel control. The so-called active regulation is conventionally carried out by thermostats controlled heaters or by embedded software coupled to thermistors. This type of regulation makes it possible to maintain the temperature of the mirror at a defined level and to compensate for the variations in flux absorbed by the front face during the year. On the other hand, this type of regulation does not make it possible to compensate for orbital fluctuations in the context of satellite in low orbit, because of the purely radiative exchange mode between the heaters and the mirror.

Other active optical type solutions exist but are expensive and complex to implement because of the use of dedicated electronics and complex functional tests on the ground, and they present a risk of failure.

The documents US-A1-2003 / 0112512 and JP-A-09133872 divulge space telescopes embedded on satellites. The document WO-A2-01 / 65297 divulge a terrestrial telescope.

Telescopes of documents US-A1-2003 / 0112512 and WO-A2-01 / 65297 comprise an inlet cavity comprising a rigid inner envelope around the primary mirror and a second envelope consisting of a thermal insulating material disposed over the entire perimeter of the cavity.

Direct thermal regulation of the active face would provide a level of stability of the equivalent mirror, but this solution is, to the knowledge of the applicant, unrealized and present risks of thermo-elastic deformation.

The present invention aims to solve this problem.

Indeed, the invention proposes a solution to the problem of very high thermal stability required at the primary mirror of an optical instrument. Its purpose is to provide a solution with respect to short-period fluctuations in the case of optical instruments from satellites of low orbit to geostationary satellites.

The proposed solution consists in producing an input cavity of the instrument comprising a rigid envelope creating a thermal inertia of all or part of the cavity.
The thermal inertia of this envelope, located in the immediate vicinity of the primary mirror where the radiative exchanges are the most important, limits the temperature fluctuations of the cavity and, consequently, the temperature fluctuations of the mirror.

The present invention more particularly relates to an optical instrument comprising at least one mirror called primary mirror (3), placed in a cavity (2), the primary mirror having an active face capable of being subjected to instantaneous variations of the radiative flux incident, where the cavity comprises a rigid inner envelope (20) around the mirror constituting at least a portion of the cavity, the envelope being made of a material having a thermal inertia so as to dampen the instantaneous variations of the incident radiative flux thus allowing to limit the temperature fluctuations of this cavity and, consequently, the temperature fluctuations of the mirror.

To limit the mass of the instrument, the rigid inner envelope extends over a first portion of the cavity defined as being close to the mirror, this portion going from the mirror to a distance d less than the total length 1 of the input cavity of the instrument.

The first envelope is made of aluminum or any other material with a high thermal inertia (eg Beryllium).

Advantageously, the aluminum casing has a thickness of about 1 mm.

According to another characteristic, the cavity further comprises a second envelope made of a thermal insulating material disposed over the entire perimeter of the cavity and at the bottom of the latter, that is to say behind the mirror.

The second envelope covers the first and extends this first cavity part for form a second part in continuity with the first ( figure 2 ).

Advantageously, the second envelope consists of an insulating multilayer structure (MLI).

According to another characteristic, the instrument further comprises active means for controlling the temperature of the mirror.

The invention applies to telescopes aboard satellites irrespective of the size of their primary mirror.

• la figure 1 représente une coupe longitudinale d'un instrument optique
• la figure 2 représente une coupe longitudinale de l'instrument optique selon un mode de réalisation.

Other features and advantages of the invention will become clear from reading the description which is given below and which is given by way of illustrative and nonlimiting example and with reference to the drawings in which:

• the figure 1 represents a longitudinal section of an optical instrument
• the figure 2 represents a longitudinal section of the optical instrument according to one embodiment.

The instrument 1 described has a cavity 2 for receiving the mirror 3, said primary mirror, and attaching it to the instrument by conventional fixing means 5. Generally the mirror is in a tubular cavity of a diameter slightly greater than its own so as to come to the periphery of this mirror. The mirror 3 is centered in the cavity and its active face turned towards the entrance of the cavity locating place of a secondary mirror 4.

In the state of the art, the cavity is made by a thermal insulation envelope made by a so-called MLI multi-layer insulating blanket (Multi Layered Insulation) painted in black on the inside of the cavity and exhibiting strong variations in temperature.

Rather than using a conventional insulation envelope, the proposed solution is to use an input cavity for the optical instrument 1 having a high thermal inertia. For this, the cavity 2 comprises at least a portion in a material with high inertia vis-à-vis rapid fluctuations in temperature. Thus, the input cavity of the instrument made in accordance with the invention is less sensitive to external fluctuations, especially with respect to rapid orbital-type fluctuations.

To this end, the input cavity 2 of the optical instrument 1 comprises a rigid inner envelope 20 around the mirror 3 made of a material having a thermal inertia damping the instantaneous variations of the incident radiative flux. The thermal inertia envelope limits the temperature fluctuations of the cavity and, consequently, the temperature fluctuations of the mirror.

The rigid envelope with thermal inertia 20 is in tubular form and forms part of the input cavity of the optical instrument 1.

On the diagram of the figure 1 the casing 20 constitutes the entire cavity 2. In this case, the length of the casing 20 corresponds to that of the inlet cavity 2. In this case, the casing goes from the primary mirror 3 to the casing 2. entrance of the cavity corresponding to the location of the secondary mirror 4.

For reasons of limitation of the mass of the optical instrument, the envelope 20 will preferably have a length less than that of the input cavity while remaining sufficiently long to ensure its damping function of instantaneous variations in the radiative flux. incident. This exemplary embodiment is illustrated by the diagram of the figure 2 , the casing constituting only part of the cavity 2.

The diameter of the envelope 20 is slightly greater than that of the primary mirror 3 so that the latter may be placed on the periphery of the mirror 3.

The cavity portion having a thermal inertia or the entire cavity having this thermal inertia with respect to the thermal fluctuations is covered with a thermal insulation jacket 21 of multilayer type "MLI".

Only part of the cavity comprises a rigid envelope with thermal inertia as represented on the figure 2 , and the insulation envelope 21 covering this cavity portion, extends over the entire length of the inlet cavity, its inner surface being in the extension of the inner surface of the thermal inertia envelope 20.

A material such as aluminum having a high heat capacity and a good thermal conductivity, can be used advantageously to achieve the thermal inertia envelope.

The inner face of the cavity portion A made of aluminum 20 is painted black for optical reasons, and the outer face is isolated from the instrument 1 with the MLI-type insulating multilayer jacket 21 in order to maintain a temperature level. low enough to regulate the mirror around 20 ° C.

1. 1) un contrôle actif 7 de la température de l'enveloppe rigide à l'aide d'une régulation de type Proportionnel-Intégral-Dérivée, par exemple, ce qui permet de diminuer encore les fluctuations thermiques de l'enveloppe et donc du miroir.
2. 2) l'association d'une régulation active du miroir en face arrière de type radiatif 6 qui devient, avec la présence de la cavité à forte inertie thermique, nettement plus efficace pour compenser des fluctuations de courte durée, de type orbitale. Ceci est du au fait que les variations instantanées du flux radiatif provenant de la cavité proche sont amorties, en raison de l'inertie de la cavité, par rapport aux variations provenant d'une cavité comprenant seulement une isolation de type MLI.
3. 3) l'association d'une régulation active du miroir en face arrière de type radiatif, dont la boucle d'asservissement est pilotée par la température de la cavité, permettant ainsi d'anticiper et de compenser les fluctuations de température du miroir.

Depending on the need, the proposed solution can be further improved with means 6 and 7 illustrated on the figure 2 , conventionally used for temperature control, namely:

1. 1) an active control 7 of the temperature of the rigid casing using a Proportional-Integral-Derivative type of regulation, for example, which makes it possible to further reduce the thermal fluctuations of the envelope and therefore of the mirror .
2. 2) the combination of an active regulation of the radiative type rear-facing mirror 6 which becomes, with the presence of the cavity with high thermal inertia, significantly more effective to compensate for short-term orbital-type fluctuations. This is because instantaneous changes in radiative flux from the near cavity are damped, due to the inertia of the cavity, compared to variations from a cavity comprising only MLI-type isolation.
3. 3) the association of an active control of the radiative type back-side mirror, the servo-control loop of which is controlled by the temperature of the cavity, thus making it possible to anticipate and compensate for the temperature fluctuations of the mirror.

• la cavité 2 étant munie d'une enveloppe rigide en aluminium 20 d'environ 1mm sur la moitié A de la longueur soit une longueur 1,2m,
• la cavité 2 étant recouverte d'une enveloppe d'isolation 21 de type MLI recouvrant l'enveloppe d'aluminium et prolongeant l'enveloppe 20 pour constituer l'autre moitié B de la cavité,

1 miroir secondaire 4
1 baffle d'entrée 10,
a permis, pour un satellite en orbite basse, de quantifier les gains obtenus selon l'invention : Structure de la cavité de l'instrument optique. Variation orbitale de la température moyenne du miroir primaire (face active) Variation orbitale du gradient thermique dans l'épaisseur du miroir primaire (mK) (mK) Classique (cavité recouverte de MLI) 310 12,4 Moitié de la cavité avec enveloppe interne rigide en aluminium, non régulée 110 4,8 Moitié de la cavité avec enveloppe interne en aluminium, régulée 90 3,6 Régulation active du miroir (type radiatif) et moitié de la cavité avec enveloppe interne en aluminium (non régulée) 16 1,7 For example, a thermal modeling of an assembly comprising:
1 primary mirror 3 of diameter 1,3m

• the cavity 2 being provided with a rigid aluminum casing 20 of about 1 mm on the half A of the length is a length of 1.2 m,
• the cavity 2 being covered with an MLI-type insulation jacket 21 covering the aluminum envelope and extending the envelope 20 to form the other half B of the cavity,

1 secondary mirror 4
1 input cabinet 10,
allowed, for a satellite in low orbit, to quantify the gains obtained according to the invention: Structure of the cavity of the optical instrument. Orbital variation of the average temperature of the primary mirror (active side) Orbital variation of the thermal gradient in the thickness of the primary mirror (MK) (MK) Classic (cavity covered with PWM) 310 12.4 Half of the cavity with rigid internal aluminum casing, unregulated 110 4.8 Half of the cavity with internal aluminum casing, regulated 90 3.6 Active regulation of the mirror (radiative type) and half of the cavity with internal aluminum casing (unregulated) 16 1.7

Thus, the modification of the structure of the input cavity of the proposed optical instrument makes it possible to attenuate the variations in incident flux seen by the active face of the mirror, and in particular the flux coming from the near cavity.

A cavity comprising an aluminum tubular casing 1.2 m long and 1 mm thick in the environment close to the primary mirror of 1.3 m in diameter is sufficient to obtain these results.

An optimization of the length of the rigid envelope is necessary according to the stability requirements requested and the increased mass generated.

Claims (4)

1. An optical instrument comprising at least one mirror, referred to as the primary mirror (3), arranged in a cavity (2), said primary mirror comprising an active surface that is susceptible to be subject to instantaneous variations of the incident radiation flux, said cavity comprising an internal rigid envelope (20) around said mirror, which constitutes at least part of said cavity, said envelope being made from material with thermal inertia, such as aluminium or any other materials with high thermal inertia, for example, beryllium, so as to absorb the instantaneous variations of the incident radiation flux, thus allowing the temperature fluctuations in said cavity and, consequently, the temperature fluctuations of said mirror to be limited, characterised in that the internal rigid envelope extends along a first part (A) of said cavity that is defined as being within the vicinity of said mirror, said part extending from said mirror up to a distance d that is less than the total length 1 of said cavity, and
   said cavity further comprises a second envelope (21) that is constituted by a thermal insulating material that is arranged over the entire perimeter of said cavity and at the bottom of said cavity, that is behind said mirror, said second envelope covering the first envelope and extending the first part (A) of said cavity so as to form a second part (B) that is continuous with the first part.
2. The optical instrument according to claim 1, characterised in that the first envelope (20) is approximately 1mm thick.
3. The optical instrument according to any one of claims 1 to 2, characterised in that the second envelope (21) is made from a multi-layer insulation structure (MLI).
4. The optical instrument according to any one of the preceding claims, characterised in that it further comprises active means (6) and (7) for controlling the temperature of the mirror and of the rigid envelope.

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